Low-cost satellite communication system

ABSTRACT

A communication system is provided that allows the use of low-cost, low-power remote terminal units that communicate substantially asynchronously and independently to a base station. To minimize cost and complexity, the remote terminal units are configured similarly, including the use of substantially identical transmission schemes, such as a common Direct Sequence Spread Spectrum (DSSS) code. To minimize collisions among transmissions, the communication system is designed to use a high-gain antenna with a limited field of view, to limit the number of cotemporaneous, or overlapping transmissions that are received at the base station. To cover a wide area, the limited field of view is swept across the area of coverage. To overcome potential losses caused by collisions, the remote terminal units are configured to repeat transmissions; to minimize repeated collisions, the repeat interval and/or duration is randomized.

This application is a continuation-in-part of copending U.S. patentapplication Ser. No. 09/045,970, filed 21 Mar. 1998.

FIELD OF THE INVENTION

This invention relates generally to satellite communication systems, andin particular to a communication system for transmitting short durationmessages from low-cost remote terminal units dispersed over a widegeographic area.

BACKGROUND OF THE INVENTION

There is a growing need for receiving information from sources that aredispersed about a wide area. For example, for efficient farming andirrigation, knowledge of soil moisture content at various locationsacross a field or region is important; for efficient shipping andnavigation, knowledge of sea conditions at various locations across anocean is important. Similarly, there is a growing need for the controlof devices that are dispersed about a wide area, such as switches thatopen or close irrigation flues. There is also a growing need forreceiving information about the location of resources that may travelabout a wide area. For example, for efficient wildlife or herdmanagement, knowledge of the location of members of the herd isimportant; for property recovery, the knowledge of the location of astolen vehicle is important. Such information and control messages arecharacterized as being of relatively short duration, and/or notnecessarily time-critical. The information content of a particularmessage may also be relatively insignificant, but the aggregateinformation content from a plurality of remote sensors, such asbarometric sensors, may have significance. However, because thesecontrol devices and information sources are dispersed over a large area,the communication of these relatively short and somewhat non-criticalmessages is often cost prohibitive using conventional communicationsystems. The use of relatively complex devices, such as cellulartelephones or pagers, to communicate the messages also make thecollection or distribution of these messages cost prohibitive.

Satellites offer the possibility of providing communications to and fromremote terminal units over a wide service area, without theinfrastructure typically required for ground-based communicationssystems. Because of the desire to keep the complexity of each satelliteto a minimum, satellites also provide the opportunity to providecommunication services customized to an anticipated type of informationtransfer. That is, a satellite communication system optimized for aparticular type of message transfer, such as a high-volume oflow-priority short-messages, will be significantly less complex, andtherefore more inherently reliable and less costly than conventionalsystems designed for high-priority continuous information transfer.

To communicate via a satellite, the transmitted signal from a groundstation must be received at the satellite at a sufficient signal tonoise ratio (SNR), and the retransmitted signal from the satellite mustbe also be received at the intended ground station at a sufficient SNR.The SNR can be increased by increasing the power density of the signalbeing received, or by reducing the power density of the noise beingreceived. To optimize the received power density, directional antennasare used to narrow the transmission beamwidth, thereby increasing theportion of the transmitted power being received by the receiver byminimizing the dispersion of the transmitted power. Because thetransmitted power density within a narrow-beamwidth antenna's beamwidthis increased, as compared to the transmitted power density from anomnidirectional antenna, a narrow-beamwidth antenna is termed ahigh-gain antenna; a wide-beamwidth antenna is termed a low-gainantenna. Directional, high-gain antennas are used to narrow the receiverbeamwidth, to decrease the portion of noise energy being received.Directional high-gain antennas must be aimed so that the intendedreceiver antenna or transmitter antenna is contained within the narrowedbeamwidth. The narrower the beamwidth, the more precise the aiming mustbe. The area encompassed by an antenna's beamwidth is termed theantenna's field of view.

To minimize the number of satellites needed to provide communicationsover a wide geographic area, each satellite should have an antenna witha field of view that covers a maximum amount of the satellite's servicearea. That is, each satellite should have a relatively low-gainwide-beamwidth antenna. To provide a high signal to noise ratio forcommunications to and from the satellite, the ground station uses ahigh-gain narrow-beamwidth antenna, to compensate for the satellite'slow-gain antenna. Satellites that broadcast television signals, forexample, utilize a relatively wide-beamwidth antenna covering theirentire service area, and each television receiver requires a high-gainnarrow-beamwidth antenna that is aimed at the transmitting satellite.

The requirement to aim a directional high-gain antenna at a satellite isinfeasible or impractical for mobile ground terminals, or for satellitesthat are moving relative to the ground terminal. This requirement alsoincreases the cost of the ground terminals, making their use forrelatively infrequent and low-priority messages cost prohibitive. Theaforementioned satellite television broadcast system uses geo-stationarysatellites, and is intended for fixed reception sites. Geo-stationarysatellites are significantly more expensive to launch and maintain thanlower altitude satellites, and, being farther from the earth, requiremore transmitted power or higher-gain antennas. A typical solution formobile ground terminals and moving satellites is to use a narrow-beamhigh-gain antenna at the satellite, and allow wide-beam antennas at theground terminals. The use of narrow-beam antennas, however, requires asignificant increase in the number of satellites needed to providecommunications over a large geographic area, because each antenna'sfield of view is significantly smaller than the satellite's servicearea, and overlapping satellite service areas are required to providesufficient fields of view that cover the geographic area. As with groundcommunications systems, however, providing a significant number ofsatellites to a sparsely populated geographic area may not beeconomically feasible, and the cost of providing such a service to aneconomically disadvantaged region may preclude its use. Furthermore, inpopulated areas, the profusion of mobile telephony and high speed datatransfer communications imposes significantly complex designrequirements on all transmitters, such that the cost of using existingsystems for the transmission of relatively short bursts of informationor control messages is not justified.

SUMMARY OF THE INVENTION

A need exists for a satellite communications system for transmittinginformation messages of relatively short duration from remote terminalunits dispersed over a wide geographic area that utilizes a minimumnumber of satellites yet allows for the use of a wide-beamwidth antennaat the remote device. There is also a corresponding need fortransmitting relative short duration control information to remoteterminal units using a minimum number of satellites and a low-gainwide-beamwidth antenna at the remote device. The remote devices shouldalso require minimal power, allowing for their use as portable or mobiledevices, and should be of minimal cost and complexity, allowing fortheir use in a wide variety of multi-point data collection activities.

These needs, and others, are satisfied by providing a communicationsystem that allows the use of low-cost, low-power remote terminal unitsthat communicate substantially asynchronously and independently to abase station. To minimize cost and complexity, the remote terminal unitsare configured similarly, including the use of substantially identicaltransmission schemes, such as a common Direct Sequence Spread Spectrum(DSSS) code. To minimize collisions among transmissions, thecommunication system is designed to use a high-gain antenna with alimited field of view, to limit the number of cotemporaneous, oroverlapping transmissions that are received at the base station. Tocover a wide area, the limited field of view is swept across the area ofcoverage. To overcome potential losses caused by collisions, the remoteterminal units are configured to repeat transmissions; to minimizerepeated collisions, the repeat interval and/or duration is randomized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a satellite service area and a field of view of ahigh gain antenna in accordance with an aspect of this invention.

FIG. 2 illustrates an embodiment of a satellite communications system inaccordance with an aspect of this invention.

FIG. 3 illustrates a timing diagram of a satellite communications systemin accordance with an aspect of this invention.

FIG. 4 illustrates a block diagram of an embodiment of a remote terminalunit and satellite communications system in accordance with an aspect ofthis invention.

FIG. 5 illustrates a block diagram of the preferred embodiment of aremote terminal unit in accordance with an aspect of this invention.

FIG. 6 illustrates a block diagram of another embodiment of a remoteterminal unit in accordance with an aspect of this invention.

FIG. 7 illustrates a block diagram of an embodiment of a receive-onlyremote terminal unit in accordance with an aspect of this invention.

FIG. 8 illustrates a block diagram of an embodiment of areceive-transmit remote terminal unit in accordance with an aspect ofthis invention.

FIG. 9 illustrates a block diagram of an embodiment of a multiplexedsatellite in accordance with an aspect of this invention.

FIG. 10 illustrates a block diagram of an embodiment of a satellitecommunication system that uses FDM in accordance with an aspect of thisinvention.

FIG. 11 illustrates a block diagram of an embodiment of a satellitecommunication system that uses CDMA/DSSS in accordance with an aspect ofthis invention.

FIG. 12 illustrates a block diagram of an embodiment of a ground stationthat uses multiple DSSS correlators having the same DSSS code inaccordance with an aspect of this invention.

FIG. 13 illustrates a block diagram of an embodiment of a satellitecommunication system that uses CDMA for transmission and reception inaccordance with an aspect of this invention.

DETAILED DESCRIPTION OF THE INVENTION

In general, the satellite communications system of this inventionprovides high-gain coverage to a wide geographic area with a minimumnumber of satellites, and allows for the use of inexpensive remoteterminal units for communicating with the satellite. A satellite inaccordance with this invention includes a high-gain antenna thatperiodically sweeps the satellite's service area to receive the messagesfrom remote terminal units within the entire service area. In apreferred embodiment, a satellite may contain multiple high-gainantennas, each antenna sweeping a portion of the entire service area.For ease of understanding, the invention is presented herein using theparadigm of a satellite that has a single high-gain antenna. Because thehigh-gain antenna sweeps the satellite's entire service area, there isno need to deploy multiple satellites with overlapping service areas. Inorder to provide high gain, the antenna is designed to have a narrowbeamwidth in at least one dimension. To cover the entire area, theantenna's field of view is swept across the entire service area. In thepreferred embodiment, the antenna's field of view is essentiallyrectilinear, having a narrow beamwidth in one dimension and a beamwidththat extends across the entire service area in the other dimension, suchthat the sweeping effect is akin to that of a common push-broom. Becauseof the high gain of the satellite antenna, communications via thesatellite can be accomplished using low power and/or using a low-gainwide-beamwidth antenna at the remote terminal unit. The system isoptimized for the use of remote terminal units that transmit shortduration messages relatively infrequently. To providecost-effectiveness, the preferred remote terminal units have minimalcapabilities, having for example a fixed transmit frequency or a fixedmodulation scheme. Because the messages are infrequent and short, thesame communication channel may be shared among multiple remote terminalunits, allowing for the mass production of virtually identical low-cost,single-purpose transmitters.

For convenience in terminology, the satellite communications system willbe described herein as including remote terminal units (RTUs) and aground station. The RTUs are the satellite communication devices thatoperate at low power and/or with low-gain antennas. The remote terminalunits may include a transmitter, a receiver, or both. The RTUtransmitter is constrained to be low powered, either to allow forsustained portable operation, or because of regulatory constraints, suchas FCC limits. Although one ground station is used in the examplesherein, the principles presented can be readily applied to multipleground stations. The ground station may operate at high power and/orwith a high-gain antenna and satellite tracking capability. However, aswould be evident to one of ordinary skill in the art, the ground stationmay also operate at low power and/or with a low-gain antenna, and may infact be similar in structure and design as the remote terminal units.Similarly, some remote terminal units may be located at sites providingvirtually unlimited power, and/or at fixed sites that allows for the useof a higher-gain antenna.

FIG. 1 shows a map of the earth with a satellite service area 100overlaid upon a portion of the earth's surface. The service area 100 isthe area on the earth's surface to and from which effectivecommunications with the satellite can be maintained. The service area100 of a satellite is determined primarily by the satellite's altitudeabove the earth, and the minimum elevation angle from the earth to thesatellite for effective communications. Although a satellite's servicearea 100 may theoretically extend to an entire hemisphere,communications to and from regions on the fringe of the theoreticalextent exhibit a significant amount of ground based interference,because signals travelling at a low elevation angle must traverse alonger distance over the earth's surface. In general, an elevation angleof at least 30 degrees is preferred. For a “low-earth” (LEO) satellite,the service area 100 is about +/−30 degrees longitude and latitude. Asthe satellite moves relative to the earth's surface, the satellite'sservice area moves as well.

FIG. 1 also shows a field of view 110 representative of an embodiment ofa high gain antenna in accordance with this invention. As shown, thefield of view 110 is substantially smaller than the satellite's servicearea. The size of the field of view 110 and the antenna gain aredirectly related. An antenna having a gain of ten over an antenna thatencompasses the entire service area 100 will have a field of view 110 ofone-tenth the area of the service area 100. In the preferred embodiment,the antenna's field of view 110 is essentially rectilinear and extendsacross the service area 100 in one dimension, although otherconfigurations would be evident to one of ordinary skill in the art.

FIG. 2 shows an illustration of a satellite 200 having a high-gainantenna 210 that has a field of view 110 within the satellite's servicearea 100. The dashed lines 112 a and 112 b indicate the bounds of theservice area 100, formed by the sweeping of the field of view in thedirection indicated by the arrow 225. The field of view 110 can be madeto sweep the service area 100 in a variety of manners. The preferredembodiment uses satellites that travel in an orbital plane. As thesatellite 200 traverses the sky above the service area 100 in thedirection indicated by the arrow 220, the field of view 110 willtraverse the path 112 a-b. Additionally, to sweep the service area moreoften than once per orbital period, the satellite 200 and antenna 210may be rotating, as shown by arrow 221, or rocking back and forth, asshown by arrows 222 and 223. Other means of having the field of view 110sweep the service area 100 would be evident to one of ordinary skill inthe art. The high-gain antenna 210 can be fixedly mounted to thesatellite 200, or movably mounted; for ease of discussion, a movement ofthe satellite 200 implies a corresponding movement of the field of view110 of the antenna 210, although an independent movement of the antenna210 may be used to effect the same result. For example, the antenna 210can be mounted as a pendulum, thereby providing the rocking motions 222and 223 with minimal energy demands to sustain the motion. Each of themeans of sweeping the service area 100 can be used independently or inconjunction with each other to effect the sweep. For example, thesatellite could be travelling in direction 220 and rotating 221 at thesame time. If the service area 100 is both wider and longer than thefield of view, the field of view 110 may be swept across the servicearea in two dimensions, for example by travelling in direction 220 whilerocking about an axis coincident with the direction of travel 220, asshown by arrow 223.

Also shown in FIG. 2 are a plurality of remote terminal units RTU 250,and a ground station 280. The RTUs 250 a and 250 b are shown to bewithin the field of view 110 of the high gain antenna 210, whereas theother RTUs 250 are outside the field of view 110. The RTUs 250 a and 250b are the only RTUs 250 that are able to communicate with the satellite200 via the antenna 210, because of the antenna's narrow beamwidth 214.Because the antenna's beamwidth is narrow, its gain is high, therebyallowing the use of a low powered transmitter and low gain antenna atthe RTUs 250 a and 250 b within its field of view 110. The required gainof the antenna 210 is determined based upon the transmitted power of theRTUs 250 a-b, the distance of the satellite 200 from the RTUs 250 a-b,the gain, if any, of the antennas at the RTUs, and the predicted noiselevel, using conventional “link-margin” calculations common to one ofordinary skill in the art. The required gain of the antenna 210thereafter determines the total beamwidth of the antenna 210, from whichthe beamwidths in each dimension can be chosen. In a typicalconfiguration, the RTUs 250 are limited to be one-watt transmitters withomidirectional or hemispherical antenna patterns, with a gain of 0 to 5dBi, where dBi is the gain relative to an isotropic antenna. To receivethe signal with a sufficient SNR at a satellite receiver located 2500 kmabove the earth, the antenna gain should be at least 14 dBi. An antennahaving a total beamwidth of 10 degrees by 90 degrees will provide a gainof approximately 14 dBi. This link margin analysis is based on a 1200baud signal at a worse case slant range at 10 degrees of elevation, anda corresponding transmitter to satellite distance of 8152 km. As wouldbe evident to one of ordinary skill in the art, increasing or decreasingthe transmission data rate will correspondingly increase or decrease therequired gain.

By sweeping the service area 100 with the field of view 110 of thehigh-gain antenna 210, each of the RTUs 250 will be within the field ofview 110 at some time, and will thus be able to transmit to thesatellite using a low powered transmitter and a low gain antenna. In asimilar manner, whenever an RTU 250 is within the field of view 110 ofthe high-gain antenna 210, it will be able to receive communicationsfrom the satellite 200 using a low gain antenna. To provide at least a2:1 gain at the high-gain antenna 210 compared to an antenna having afield of view equal to the satellite service area 100, the field of view110 should be less than half the service area 100. As the field of view110 is reduced relative to the service area, the transmit power andantenna gain requirements at the RTU 250 decrease.

The ground station 280 is shown having a directional high-gain antenna282. Because the ground station uses a high-gain antenna 282, thesatellite is able to use a wide-beamwidth low-gain antenna 240 forcommunications with the ground station 280. By using a wide-beamwidthantenna 240 for communicating with the ground station 280, the groundstation 280 can communicate with the satellite independent of the fieldof view 210 of the high-gain antenna 210. That is, the ground station280 can be anywhere within the field of view of the wide-beamwidthantenna 240. As in a conventional satellite system, the satellite isdesigned with minimal transmit power requirements. Link-margincalculations determine the required gain of the ground station antenna282 to allow for the minimal satellite requirements, balanced againstthe cost of providing the accurate satellite tracking required for ahigh-gain antenna. Alternatively, if the satellite 200 uses the samehigh-gain antenna 210 to communicate with the ground station 280, theground station 280 need not use a high-gain antenna that requiressatellite tracking. U.S. Pat. No. 6,128,469 “Satellite CommunicationSystem with a Sweeping High-Gain Antenna, issued 3 Oct. 2000, to RaymondG. Zenick Jr., John Eric Hanson, Scott A. McDermott, and Richard D.Fleeter is incorporated by reference herein. Disclosed in thisreferenced patent are a variety of configurations for effecting theabove communication scheme. Of particular note, the satellite 200 may beconfigured with a plurality of antennas that are electronically switchedto effect a sweeping pattern. In such an embodiment, the order ofselection of the antenna can be arbitrary. For the purpose of thisinvention, the term sweep is interpreted to include any time-sequentialscanning, or ‘illumination’, of smaller areas 110 within the coveragearea 100. In like manner, the term antenna is interpreted to include asingle antenna, as well as any currently active antenna, or plurality ofantenna elements, among a plurality of antennas.

FIG. 3 shows the timing relationships introduced by the sweeping of aservice area 100 by a high gain antenna 210. The time during which anRTU 250 is within the field of view 110 of the high-gain antenna 210 istermed the illumination period 850. The overall time during which thehigh-gain antenna 210 is sweeping the service area 100 is theillumination sweep period 810. The time duration between the start ofeach sweep is the sweep period 800, and the difference between the sweepperiod 800 and the illumination sweep period 810 is the nonilluminationperiod 815.

Each RTU 250 has an illumination period 850 that is substantially lessthan the sweep period 800, and in particular, substantially less thanthe illumination sweep period 810. The ratio of an RTU's illuminationperiod 850 and the illumination sweep period 810 is dependent upon thegain of the high-gain antenna 210, and, correspondingly, the size of thefield of view 110 relative to the service area 100. As discussed above,the field of view 110 should be less than half the size of the servicearea 100. In a typical embodiment, the field of view 110 is less than afifth of the service area 100, and thus, an RTU's illumination period850 will be less than a fifth of the illumination sweep period 810. Thisprovides a 5:1 improvement in antenna gain, compared to an antennahaving a field of view equal to the service area 100. The location ofthe RTU illumination period 850 relative to the illumination sweepperiod is dependent upon the particular RTU's 250 location within theservice area 100, relative to the sweep 225 of the field of view 110 ofthe high-gain antenna 210.

Because an RTU 250 is not continually within the field of view 110 ofthe high-gain antenna 210, each RTU 250 must be designed so as to assurethat the intended message is transmitted when the RTU 250 is illuminated850 by the high gain antenna 210. FIG. 4 illustrates an embodiment of anRTU 250 that responds to trigger signals 872 transmitted from thesatellite 200. The RTU 250 includes a message source 910, a receiver930, and a transmitter 920. The message source 910 may be a transducerthat is measuring some parameter, such as soil moisture content, or anyother device that generates an information message 915 intended to betransmitted. Upon receiving a trigger signal 872, the receiver 930issues a trigger pulse 874 to the transmitter 920. Upon receiving atrigger pulse 874 from the receiver 930, the transmitter receives theinformation message 915 from the message source 910, and transmits acorresponding transmission message 860. The satellite receiver 310receives the transmission message 860 via the high gain antenna 210. Inorder for this system to operate properly, the transmission message 860must be transmitted when the RTU 250 is within the field of view 110 ofthe high-gain antenna 210. As shown in FIG. 4, the satellite 200includes a trigger transmitter 370 that transmits trigger signals 872via the high-gain antenna 210. The antenna diplexor 378 decouples thereceiver 310 from the high-gain antenna 210 during the brief periods oftransmission of the trigger signals 872. Example trigger signals 872 areshown on line 3C of FIG. 3. Because these trigger signals 872 aretransmitted via the high-gain antenna 210, the RTU 250 of FIG. 5 willnot receive the trigger signals 872 until the RTU 250 is within thefield of view 110, shown by the illumination period 850 in FIG. 3. Thereceiver 930 of the RTU 250 will produce the trigger pulse 874corresponding to the first trigger signal 872 that occurs within theillumination period 850. As shown on line 3C, in response to thistrigger pulse, the transmitter 920 transmits the transmission message860 immediately after the first trigger signal 872 that occurs withinthe illumination period 850. Because the generation of the trigger pulse874 cannot occur until the RTU 250 is within the field of view 110 ofthe high-gain antenna 210, and the transmission occurs immediatelythereafter, the transmission message 860 will be received via thehigh-gain antenna 210. Note, however that the duration 862 of thetransmission message 860 cannot exceed the illumination period, else thetrailing end of the transmission message 860 will occur when the RTU 250is no longer within the field of view 110 of the high-gain antenna 210.Also, because the trigger signal 872 is asynchronous with theillumination period of each particular RTU 250, the sum of the period ofthe trigger signals 872 and the message duration 862 must be less thanthe illumination period 850 in order to assure that the transmissionmessage 860 is completed before the end of the illumination period 850.

To support the transmission of relatively long information messages, thetransmitter 910 can be configured to partition the information message915 into partial information messages, and transmit each of the partialinformation messages as a transmission message 860 having a messageduration 862 that conforms to the above constraint. Also, thetransmission of messages from an RTU 250 can be further optimized bychecking each information message 915 with its immediately priorinformation message, and only transmitting a transmission message 860when the there is a difference from one message to the next.

Note that the RTU 250 of FIG. 4 includes a receiver 930, and thesatellite 200 includes a trigger transmitter 370 and diplexor 378. Inaddition to the additional cost introduced by these components, afailure in either of these components will preclude communications fromthe RTU 250, and a failure of the trigger transmitter 370 or diplexor378 will preclude communications to the satellite 200 from all RTUs 250.The preferred embodiment of FIG. 5 shows an RTU 250 for use with asatellite 200 as shown in FIG. 3; that is, the preferred embodiment doesnot require the use of a trigger signal to effect communications.

In FIG. 5, the RTU 250 includes a message source 910, a storage element950, a timer 940, and a transmitter 920. The message 915 from themessage source 910 is stored in the storage element 950; this message isaccessible as required by the transmitter 920 via 918. The messagesource 910 also generates an event flag 912. The event flag 912, forexample, may be a flag that is asserted whenever consecutivemeasurements of a parameter differ by a specified amount, or whenever aparameter exceeds a particular value. The event flag 912 initiates thetransmission of a transmission message 860 corresponding to the message915 that is stored in storage element 950. The event flag 912 alsoactivates the timer 940. The timer 940 is an interval timer that assertsa duration signal 942 coupled to the transmitter 920. In accordance withthe preferred embodiment of this invention, the transmitter 920repeatedly transmits the transmission message 860 corresponding to thestored message 915 while the duration signal 942 is asserted. Becausethe operation of the RTU 250 of FIG. 5 is independent of a trigger orsynchronization signal from the satellite 200, the duration signal 942must be asserted for a repetition duration 866 that will encompass theillumination period 850, as shown at line 3D of FIG. 3. In theembodiment depicted at line 3D, the repetition duration 866 exceeds thesweep period 800, thereby assuring that at least one illumination period850 is included in the repetition duration 866. Also, the time duration864 between the start of one transmission message 860 and the end of thenext transmission message 860 is less than the illumination period 850,thereby assuring that at least one full transmission message 860 iscontained within the illumination period 850. Thus, by providing for astore-and-retransmit capability at the RTU 250, the RTU 250 reliably andeffectively communicates via a satellite 200 without requiringsynchronization or coordination means between the RTU 250 and thesatellite 200. In the preferred embodiment, the repetition duration 866is kept to near minimum, to reduce power consumption by the RTU 250. Theminimum repetition duration 866 is slightly less than the sweep period800; in the preferred embodiment, the repetition duration 866 is betweenone and two sweep periods 800, to provide a sufficient margin of error.Also preferably, to reduce the likelihood of repeated collisions amongtransmitters, the repeat interval 865 and/or the repeat duration 866 arevaried, preferably via a random process. For the purposes of thisapplication, the term “random” includes any process wherein the use ofthe same interval or duration for a series of repeated transmissionsduring a common time period by two RTUs 250 is highly unlikely. Forexample, each RTU 250 may include a free-running counter, and the repeatduration 866 and/or interval 865 is based on the value of one or morebits of the counter when the transmission commences. Alternatively, toreduce power requirements, each RTU 250 may contain a static parameter,such as an identifier, or source address, of the particular RTU 250, andthe repeat duration 866 and/or interval 865 is based on one or more bitsof this static parameter. Other techniques for providing differingdurations 866 and/or intervals 865 in some or all of the RTUs 250 willbe evident to one of ordinary skill in the art in view of thisdisclosure.

Also shown in FIG. 5 is a power source 980. The power source 980 may bea conventional portable or fixed power supply, such as a battery or ACsupply. Solar cells and other forms of power sources may be used aswell. For example, in the triggered embodiment shown in FIG. 4, thereceiver 930 can be a passive resonant circuit that is excited by thetrigger signal 872 from the high-gain antenna 210. The energy inducedinto the resonant circuit by the transmitted trigger signal 872 can beused to subsequently activate and power the transmitter 920, similar tothe concept used to induce the transmission of information from passivedevices such as ID cards that are read from a distance.

Other embodiments of an RTU 250 consistent with this invention will beevident to one of ordinary skill in the art. For example, FIG. 6 showsan alternative embodiment that uses the principles presented in thepreferred embodiment of FIG. 5. In FIG. 6, the timer 940 generates boththe duration signal 942, as well as the event flag 912. Such anembodiment would be used, for example, to generate periodic messages,rather than messages based on the source of the message. The messagesource 910 may be, for example, a Global Positioning System (GPS)receiver that generates the RTU's 250 global location coordinates. Thetimer 940 may generate an event flag every hour, thereby providing anhourly report of the RTU's 250 location, for the tracking of mobile RTUs250, such as livestock or vehicles. Also, absent from FIG. 6 is astorage element 950. In this example embodiment, the transmitter 920receives continual messages 916 from the message source 910, forexample, continual coordinate locations, or continual soil moisturereadings. Each of the “repeated” transmission messages 860 may containdifferent information, corresponding to the continual messages. Ingeneral, the differences among the transmission messages 860 areexpected to be slight, such that the receipt of any one of thetransmission messages 860 is sufficient to convey the desired periodicinformation.

FIG. 7 shows an RTU 250 that includes a receiver 930 and an optionalcontrol device 934. The receiver 930 may be used to receive, forexample, text or paging messages at a remote location. The optionalcontrol device 934 may be coupled to the receiver 930 for receivingcontrol messages, for example to control a switch or a valve, or tosound an alarm. FIG. 8 is a composite of FIGS. 6 and 7, wherein thereceiver 930 is operably coupled to the timer 940 that controls thetransmitter 920. In this example embodiment, the duration signal 942 isasserted until a confirmation 936 is received that the informationmessage 915 has been received at the satellite 200 or ground station180.

FIG. 9 illustrates an example block diagram of a satellite 200 that isconfigured to use common equipment for both the uplink 201 and downlink202 communications to and from the satellite 200. In this configuration,independence between uplink 201 and downlink 202 communications ismaintained via a time-sharing of the equipment, rather than the use ofduplicate equipment. At one time interval, as determined by a controller340 and multiplex switch 211, the receiver 311 receives communicationsfrom the RTUs 250 via the high gain antenna 210, and at another timeinterval, it receives communications from the ground station 280 via theuplink antenna 212. Similarly, the downlink antenna 240 and thewide-beamwidth antenna 242 are time-division multiplexed 241 at theoutput of a common transmitter 331. As would be evident to one ofordinary skill in the art, if the antennas 240 and 242 have similar gainrequirements, the switch 241 and either one of the antennas 240 or 242can be eliminated.

FIG. 3 shows an example of timing diagrams corresponding to themultiplexed embodiment of a satellite communication system shown in FIG.8, at lines 3E and 3F. During the illumination sweep period 810, thereceiver 311 is enabled 820 to receive communications from the RTUs 250;during the nonillumination period 815, the receiver 311 is enabled 825to receive communications from the ground station 280. In this manner,the same receiver 311 is used to perform the function of the receivers310 and 312, and the same frequency can be used for all uplinkcommunications to the satellite. Similarly, the transmitter 331 isenabled 830 during the illumination sweep period 810 to transmit to theground station 280, and the transmitter 331 is enabled 835 during thenonillumination period 815 to transmit to the RTUs 250, thereby allowingone transmitter and one frequency to be used for all downlinkcommunications from the satellite.

In the preferred embodiments, the satellite 200 and RTUs 250 will be ofminimal complexity, thus maximizing the satellite's reliability, andminimizing the RTUs 250 costs. The satellite 200 receives a radiofrequency (RF) bandwidth of information at one frequency, andretransmits the same RF information bandwidth to the ground station 280at a second frequency. All demodulation and decoding is preferablyperformed at the ground station 280. Similarly, all messages being sentto the RTUs 250 are encoded and modulated at the ground station 280 andtransmitted to the satellite 200 at one frequency and retransmitted tothe RTUs 250 at another frequency. As discussed with regard to lines 3Eand 3F of FIG. 3, by multiplexing the function of the uplink receiver311 and downlink transmitter 331, the bandwidth about one frequency, theuplink center frequency, can be used by either the RTUs 250 or theground station 280 for transmission to the satellite 200, and thebandwidth about another frequency, the downlink center frequency, can beused by the satellite 200 to transmit to either the RTUs 250 or theground station 280. For ease of discussion, this single uplink frequencyand single downlink frequency model will be used hereinafter.

As a further cost reduction measure, the satellite 200 of FIG. 9 isillustrated as containing an RTU 250′ that is coupled to the spacecraftcontrol system of the satellite 200. Conventionally, the spacecraftcommand and control communications are provided by a communications pathto the ground station 280 that is independent of the payloadcommunications (receiver 311 and transmitter 331). By placing an RTU250′ within the spacecraft, and configuring the ground station 280 andsatellite 200 to communicate the required command and controlinformation using the principles presented herein for communicating toand from an RTU 250 in accordance with this invention, a separatespacecraft communications path is not required. The RTU 250′ differsfrom the typical RTUs 250 of this invention only in the means ofreceiving and transmitting the communications. The typical RTU 250 usesone or more antennas for remotely communicating with the transmitter 331or receiver 311, whereas the RTU 250′ is coupled directly to thetransmitter 331 and receiver 311, using techniques common in the art.For example, with reference to FIG. 8, the input of the receiver 930 ofthe RTU 250′ is coupled to the output of the receiver 311 via ahigh-impedance isolation device, and the output of the transmitter 920of the RTU 250′ is coupled to the input of the transmitter 331 via acommon adder circuit.

The bandwidth allocated for communicating the messages from the RTUs 250to the ground station 280 must be sufficient to accommodate some maximumnumber of RTUs 250 communicating to the ground station 280 at the sametime. This bandwidth is common to both the uplink and downlink paths tothe satellite 200. Each RTU 250, however, does not require the entirebandwidth. The RTUs 250 can use any number of transmission modulationschemes to utilize the available bandwidth.

FIG. 10 shows an embodiment of a satellite communications system thatutilizes a frequency division allocation, or multiplexing (FDM), of theavailable bandwidth BW. The satellite 200 transforms an uplink signal201, which is the RF information bandwidth centered about a frequencyF0, into a downlink signal 202 that is the same RF information bandwidthcentered about a different frequency F 1. Shown in FIG. 10 is the use offive different frequencies f1, f2, . . . f5 for communication from theRTUs 250 to the ground station 280 via the satellite 200. Each of thefrequencies f1, f2, . . . f5 lie within the RF information bandwidth BWcentered about a frequency F0, that is, within the uplink signal 201.Each RTU 250 is allocated one of the five transmission frequencies.Those allocated to frequency f1 are identified as RTUs 251; thoseallocated to frequency f2 as 252; frequency f3 as 253; frequency f4 as254; and frequency f5 as 255. The ground station 280 includes a widebandreceiver 284, capable of receiving the downlink signal 202, which is theRF information bandwidth BW centered about F1, from the satellite 200.The wideband receiver 284 includes receiver components 286 thatsegregate the received bandwidth BW into segments corresponding totransmission frequencies f1, f2, . . . f5. The receiver components 286produce forwarding messages 287 that are processed by a router 288 andforwarded as destination messages 289, as will be discussed below.

In the embodiment of FIG. 10, each of the RTUs 251 transmits atfrequency f1; these transmissions will be detected at 291, the receivercomponent 286 corresponding to frequency f1. If more than one RTU 251transmits a transmission message that is received at the satellite 200at the same time, the detected transmission at 291 will be, in general,a distorted combination of the received transmissions from each of theRTUs 251, and will be unusable. As noted above, however, the satellitecommunication system in accordance with this invention is preferablyused for the communication of relatively short duration and infrequentinformation messages. Therefore, the likelihood of two RTUs 251transmitting an information message at the same time is relatively low.Furthermore, the field of view 110 of the high-gain antenna 210 that isused to receive the transmission messages from the RTUs 251 issubstantially smaller (at least half) the satellite service area 100.Therefore, assuming a somewhat random distribution of RTUs 251, thetransmissions of at least half the RTUs 251 within the service area 100will not be received by the satellite 200 at any given time, therebyreducing the likelihood of the reception of overlapping signals frommore than one RTU 251 at the same time. This same assessment of thelikelihood of overlapping receptions by the satellite 200 can be appliedto transmissions from RTUs 252, 253, 254, and 255. As the bandwidth BWis increased, the number of transmission frequencies f1, f2, . . . fnallocated among the RTUs 250 can be increased, thus further reducing thelikelihood of a collision, i.e. the reception of overlappingtransmissions from more than one RTU 250 operating at the samefrequency. As noted above, the preferred embodiment of the invention isintended for relatively low-priority messages, such that the loss of amessage due to a collision is not catastrophic. As would be evident toone of ordinary skill in the art, however, if a particular RTU 250 isrequired to be collision free, the frequency assigned to that particularRTU 250 can be restricted, such that no other RTU 250 within the fieldof view 110 of the high-gain antenna 210 of the satellite 200 isallocated that same frequency. Similarly, unique frequencies may beassigned for transmissions to RTUs 250 that contain a receiver 930, sothat the transmission of messages from other RTUs 250 in the vicinity ofreceiver 930 will not interfere with the reception of messages from theground station 280. Preferably, for example, if an RTU 250′ is used inthe spacecraft for command and control, as illustrated in FIG. 9, thisRTU 250′ will be allocated a transmit and receive frequency that differsfrom all other RTUs 250, to assure collision-free communications.

FIG. 11 shows an embodiment of a satellite communications system thatuses a Code Division Multiple Access (CDMA) transmission protocol. As ina typical CDMA system, this communication system uses Direct SequenceSpread Spectrum (DSSS) modulation scheme. A DSSS modulation is a linearmodulation of a carrier frequency in accordance with a particular DSSScode value, typically via a binary phase shift key (BPSK) or similarmodulation, such as PAM, QPSK, OQPSK, and MSK. Each transition of theparticular DSSS code value introduces a phase shift of the carriersignal. A correlator at the receiving end applies an inverse of the sameDSSS code value to the received signal; if the decoded result shows astrong correlation to an unmodulated carrier signal, the correlatorlocks onto the received signal and produces the decoded result as anoutput. If a strong correlation is not found, for example, because thereceived signal was encoded using a different DSSS code, the receivedsignal is ignored. DSSS codes that produce modulations that are eachstrongly uncorrelated with each other are termed orthogonal DSSS codes.The size, or length, of the DSSS codes is determined so as to spread themodulated carrier signal across the entire bandwidth BW. FIG. 11 showsthe use of five orthogonal DSSS codes, DSSS1, DSSS2, . . . DSSS5 in RTUs250, identified as RTUs 261, 262, . . . 263 respectively. The groundstation 280 of FIG. 11 includes a wideband receiver 285 that includesDSSS correlators 283 that produce decoded messages 287 corresponding tocodes DSSS1, DSSS2, . . . DSSS5. As in FIG. 10, the overlappingreception of transmissions that use the same DSSS code will result in acollision. However, as contrast to FDM, once a correlator 283 locks ontoa particular received signal, the occurrence of another received signalusing the same DSSS code that starts at a later time is, in general,ignored in the same way that other uncorrelated signals are ignored.This is because once the correlator 283 locks onto a signal, itmaintains a time-dependent correlated relationship with the signal,sequencing through each bit value of the DSSS code. That is, a secondreceived transmission using the same DSSS code will be ignored, but itwill not adversely affect the first received transmission.

Note however, that in the embodiments of FIG. 10 and FIG. 11, the RTUs250 are designed to have one of a fixed number of allocated frequenciesor DSSS codes. To minimize the likelihood of collisions, RTUs 250 havingthe same allocated frequency or DSSS code should be uniformlydistributed over the entire service area 100. In addition to theadministrative overhead associated with allocating particular RTUs 250to particular area, such an allocation may be impossible to enforce formobile RTUs 250. Also, the allocation of resources at the ground station280 is somewhat inefficient. If two RTUs 261 that use the same DSSS1code are within the field of view 110 of the satellite 200 transmitcoincidentally, one or both of the transmission messages will be lostdue to a collision, even if no other RTUs 262, 263, 264, or 265 aretransmitting. That is, the DSSS correlators 283 associated with DSSScodes DSSS2, DSSS3, DSSS4, and DSSS5 may be idle while messagestransmitted with a DSSS1 code are being lost.

FIG. 12 shows an embodiment of a ground station 280 that is optimized toreduce the likelihood of lost messages due to collisions. The groundstation 280 is designed to provide communications to a plurality of RTUs250 that use the same DSSS code; in this example, a plurality of RTUs250 that use DSSS1 (illustrated as reference items 261 in FIG. 11). Thewideband receiver 285 of the ground station 280 includes a downconverter 610, a controller 620, and DSSS correlators 631 through 635that use the same DSSS1 code to provide output signals 287. Thecontroller 620 is operably coupled to each correlator 631-635, toprovide a seek signal to each, and to receive a locked-on signal fromeach. The down-converter 610 extracts the RF information bandwidth BWfrom the downlink signal 202 to produce an intermediate signal 615.Initially, the controller 620 asserts the seek signal to correlator 631,and deasserts it to the other correlators 632-635. The seek signalinstructs the selected correlator 631 to enter a seek mode, to searchfor a signal within the intermediate signal 615 that is stronglycorrelated to the DSSS1 code. When correlator 631 locks onto a receivedsignal in the intermediate signal 615, it enters a locked-on mode, andnotifies the controller 620. The controller 620 deasserts the seeksignal to correlator 631, and asserts the seek signal to correlator 632.The correlator 631 proceeds to decode the correlated received signal,while the newly selected correlator 632 searches for another receivedsignal that is correlated to the DSSS1 code. Because the correlator 632is enabled for seeking after the start of the received signal that wasdetected by the correlator 631, the correlator 632 does not detect astrong correlation to this same received signal. When a second signal isreceived that is correlated to the DSSS1 code, the correlator 632 locksonto it and notifies the controller 620. Note that this receipt of asecond correlated signal by correlator 632 is independent of whether thefirst correlated received signal is still being received and decoded bythe correlator 631. Thereafter, the controller 620 deasserts the seeksignal to the correlator 632 and asserts the seek signal to correlator633, or to correlator 631 if correlator 631 deasserts its locked-onsignal, indicating that the receipt of the first correlated signal hasbeen completed. This process continues, such that the controller 620enables each available correlator to seek until all correlators areunavailable because they are each receiving and decoding a receivedsignal having a DSSS1 code. Thus, in this example embodiment, a messagewill not be lost because of a collision until all correlators are inuse, thereby optimizing the use of resources within the ground station280. In the preferred embodiment, each RTU 250 uses the same DSSS code.When the population density of RTUs 250 in a service area 100 increasesto such an extent that collisions result in lost messages, the groundstation need only be augmented to include additional correlators havingthis same DSSS code.

FIG. 13 shows an embodiment of a satellite communication system thatprovides communications to and from the RTUs 250. Transmissions from theground station 280 to the RTUs 250 that contain a receiver 930 use anorthogonal DSSS code to the RTUs 250 transmission DSSS code, to isolateeach receiver 930 from interference from transmitting RTUs 250. Asshown, in the preferred embodiment, the ground station 280 includes awideband transmitter 290 that includes multiple DSSS modulators 296. Inthe preferred embodiment, each of the DSSS modulators 296 use the sameDSSS code, shown as DSSS2 in FIG. 13. Using the same time-separated useof the same DSSS code presented above, the controller 298 enables eachDSSS modulator selectively, such that the modulations do not begin atexactly the same time, but multiple modulations can be occurring at thesame time. Each message sent from the ground station 280 will contain atarget address, identifying the address 970 associated with each RTU.Each RTU 250 having a receiver 930 will demodulate the messages beingsent from the ground station 280 and process the messages that containthe RTUs address 970 as the target address. The RTU receiver 930contains a correlator 931 that has a seek mode and a locked-on mode. Thecorrelator 931 will remain in the seek mode until it locks onto amessage from the ground station 280. If the message contains the addressof the RTU 250 as its target address, the correlator 931 will remainlocked onto the message until it ends. As soon as the message isdetermined not to contain the address of the RTU 250 as the targetaddress, the correlator 931 reenters the seek mode. The controller 298enables each DSSS modulator 296 after the transmission of the portion ofthe message that contains the target address. In this manner, each RTU250 may use the same DSSS code for the reception of messages from theground station 280, while still allowing the ground station 280 totransmit multiple messages at the same time.

As in the example frequency-division multiplexing (FDM) system of FIG.10, if a collision-free channel is required, such as for command andcontrol of the spacecraft functions 390 of the satellite 200, a uniqueDSSS code may be allocated to an RTU 250′ provide this channel. Byproviding a unique DSSS code that is orthogonal to the DSSS codes of theother RTUs 250, the communications via this channel will becollision-free, even though the channel shares the same frequency as theother RTUs 250.

The transmission messages 860 from each RTU may be conventional messagepackets, containing a source address, a destination address, and theinformation message 915 from the message source 910. The router 288 ofthe ground station 280, in FIGS. 10, 11, and 12 processes the receivedand decoded messages 287 and communicates the message to the locationcorresponding to the destination address, typically via conventionalcommunication sources, such as telephone networks, internet, or othersatellite systems.

The foregoing merely illustrates the principles of the invention. Itwill thus be appreciated that those skilled in the art will be able todevise various arrangements which, although not explicitly described orshown herein, embody the principles of the invention and are thus withinits spirit and scope. For example, although this invention isparticularly well suited for RTUs 250 that communicate information to aground station 280 for subsequent processing or communication to othersystems, the system of this invention may also be used to communicateinformation from one RTU 250 to another RTU 250. In such an application,the ground station 280, or subsequent processing system, is configuredto recognize that the destination of a message from an RTU 250 isanother RTU 250, and is configured to subsequently retransmit themessage to the other RTU 250, using the techniques disclosed above.Thus, for example, an RTU 250 may be configured to report malfunctionsof a monitored device, and the reported malfunction may be relayed to anRTU 250 that is coupled to a PDA (Personal Data Assistant), or similardisplay device, that is carried by a repairperson. In such anapplication, some RTUs 250 may be configured as receive-only devices,such as pagers and the like. In like manner, although the communicationssystem presented herein is particularly well suited for satellitecommunications, some or all of the principles of this invention may beapplied to ground-based systems as well. For example, an inexpensivetwo-way paging system can be provided wherein each pager is configuredto repeatedly transmit a confirmation message for a given duration,using a common DSSS code, and the ground-based base station isconfigured to distinguish among cotemporaneous acknowledgement messagesfrom different pagers based on the different arrival times of at leastsome of the repeated messages. These and other system configuration andoptimization features will be evident to one of ordinary skill in theart in view of this disclosure, and are included within the scope of thefollowing claims.

1-3. (canceled)
 4. A communication system comprising: a plurality ofremote terminal units, each including a transmitter that transmits atransmission message to a satellite having a service area and ahigh-gain antenna with a field of view that sweeps the service areaduring a sweep period, such that: the transmitter is within the field ofview for an illumination period that is substantially less than thesweep period, and the transmission message has a message duration thatis less than the illumination period; and, a ground station thatreceives retransmission messages corresponding to the transmissionmessage of at least a first terminal unit of the plurality of remoteterminal units when the transmitter of the first terminal unit is withinthe field of view, wherein a DSSS code of a plurality of DSSS codes isallocated to each of the plurality of remote terminal units, and thetransmitter of each of the plurality of remote terminal units isconfigured to transmit in accordance with the DSSS code allocated to theeach of the plurality of remote terminal units.
 5. The communicationsystem of claim 4, wherein: the transmitter of the first terminal unitis configured to transmit in accordance with a predetermined DSSS code,and the transmitter of a second terminal unit of the plurality of remoteterminal units is configured to also transmit in accordance with thispredetermined DSSS code.
 6. The communication system of claim 5, whereineach of the plurality of remote terminal units is configured to transmitin accordance with the predetermined DSSS code.
 7. The communicationsystem of claim 4, wherein: each of a plurality of receiving units ofthe plurality of remote terminal units also includes a receiver thatreceives control messages that are transmitted from the ground station,and the receiver of each of the plurality of receiving units includes aDSSS correlator that demodulates the control messages in dependence upona predetermined DSSS code associated with the each of the plurality ofreceiving units.
 8. The communication system of claim 7, wherein thereceiver of the each of the plurality of receiving units includes acontroller, operably coupled to the DSSS correlator, that controls theDSSS correlator in dependence upon a portion of the control messages.9-12. (canceled)
 13. A communication system comprising: a base stationthat receives an information bandwidth that includes a plurality oftransmission messages, the base station including: a plurality of DSSScorrelators for demodulating at least two of the plurality oftransmission messages based on a predetermined DSSS code, eachcorrelator of the plurality of DSSS correlators using the predeterminedDSSS code, wherein a first message of the at least two of the pluralityof transmission messages has a first start time, and an end time, asecond message of the at least two of the plurality of transmissionmessages has a second start time that is between the first start timeand the end time, at least one of the first start time and the secondstart time is independent of the base station, and the base station isconfigured to distinguish between the first and second message based ona distinction between the first and second start times.
 14. Thecommunication system of claim 13, wherein each correlator includes aseek mode and a locked-on mode, and the base station further includes acontroller that selectively controls a first correlator of the pluralityof DSSS correlators to enter the seek mode when a second correlator ofthe plurality of DSSS correlators enters the locked-on mode.
 15. Thecommunication system of claim 13, wherein each of the plurality oftransmission messages includes a source address that identifies one of aplurality of remote terminal units, and the base station furtherincludes a destination determinator that determines a destinationaddress based on the source address.
 16. The communication system ofclaim 13, wherein the base station further includes a router thatforwards a demodulated message from each correlator of the plurality ofcorrelators to a destination.
 17. The communication system of claim 13,also including: a plurality of remote terminal units, each of the remoteterminal units including a transmitter for transmitting an each of theplurality of transmission messages based on the predetermined DSSS code.18. The communication system of claim 17, wherein the transmitter of atleast one of the remote terminal units is configured to repeat each ofthe plurality of transmission messages based on a repeat parameter thatdiffers from another repeat parameter of at least one other transmitterof the remote terminal units.
 19. The communication system of claim 17,wherein transmissions from the remote terminal units to the base stationare via a satellite having a high-gain antenna with a field of view thatsweeps a service area of the satellite.
 20. A communication systemcomprising: a base station that transmits an RF bandwidth that includesa plurality of messages, the base station including: a plurality of DSSSmodulators that modulate at least two of the plurality of messages basedon a predetermined DSSS code, each modulator of the plurality of DSSSmodulators using the predetermined DSSS code, a controller thatselectively controls each of the plurality of DSSS modulators based on atarget address contained in each of the plurality of messages.
 21. Thecommunication system of claim 20, wherein transmissions from the basestation to the remote terminal are via a satellite.
 22. A satellitesystem comprising: a spacecraft control system that is configured tocontrol a behavior of the satellite system, a receiver that isconfigured to receive communications from a base station for subsequentretransmission to a plurality of remote terminal units, a transmitterthat is configured to transmit communications from the plurality ofremote terminal units to the base station, and a satellite terminalunit, operably coupled to the receiver, the transmitter, and thespacecraft control system, that is configured to: monitor thecommunications from the base station for messages that are addressed tothe spacecraft control system, communicate the communications that areaddressed to the spacecraft control system to the spacecraft controlsystem, and communicate messages from the spacecraft control system tothe base station via the transmitter.
 23. The satellite system of claim22, wherein the communications from the base station are modulated usinga plurality of DSSS codes, and the satellite terminal unit includes aDSSS demodulator that is configured to demodulate a predetermined DSSScode of the plurality of DSSS codes, and the communications that areaddressed to the spacecraft control system are modulated using thepredetermined DSSS code.
 24. The satellite system of claim 22, whereinthe communications from the remote terminal units are modulated usingone or more DSSS codes, and the satellite terminal unit includes a DSSSmodulator that is configured to modulate the messages using apredetermined DSSS code that differs from the one or more DSSS codesused by the remote terminal units.
 25. The satellite system of claim 22,wherein the receiver is configured to provide the communications fromthe base station to the transmitter, and the transmitter is furtherconfigured to provide the subsequent retransmission of thecommunications from the base station to the remote terminal units. 26.The satellite system of claim 22, wherein the receiver is furtherconfigured to: receive the communications from the plurality of remoteterminal units, and provide the communications from the plurality ofremote terminal units to the transmitter for transmission to the basestation. 27-29. (canceled)